Dimming of lighting illumination is desirable for both energy efficiency and user preference, as well as for compensating for variations in natural lighting. In existing ballast systems for powering fluorescent lamps at adjustable illumination levels, different methods are used for dimming control. One popular method of dimming control employs a phase-control device, such as a triac. The phase-control device is used to modify a firing phase angle or “on” time of each half cycle of an alternating current (AC) powering signal. A dimming ballast system, in turn, controllably dims a fluorescent lamp based on the firing phase angle.
Another popular method of dimming control is based on a direct current (DC) input, such as a 0 to 48 volt DC input, distinct from an AC powering signal. In this method, a fluorescent lamp is dimmed based on the magnitude of the voltage of the DC input.
FIG. 1 depicts a known dimming ballast system 10 for powering a fluorescent lamp 12. The system 10 includes an electronic dimming ballast 14 having two pairs of output wires, each pair being connected to respective opposite ends of the lamp 12, and a dimmer switch 16, essentially a potentiometer having a manual slide 18, connected by a pair of wires 28 to the ballast 14.
An AC to DC converter 20 converts an input AC voltage, typically 90 to 265 volts AC 50/60 Hz, to a lower DC voltage, for example, 24 volts DC. The 24 volts DC power is supplied by a pair of wires via a wall switch 24 to a contact relay 22 that is, in turn, connected by a pair of wires 30 to the ballast 14. The 24 volts DC power is also supplied by another pair of wires to a low voltage isolated power supply 26. The relay 22 is connected by a wire 32 to the dimmer switch 16 and by a wire 34 to the power supply 26. The power supply 26 is also connected by a wire 36 to the dimmer switch 16.
The dimmer switch 16 regulates the brightness level of the lamp 12 by sliding the slide 18 between high and low position levels. When the slide 18 is set at its lowest level, the relay 22 and the power supply 26 cooperate to turn off the ballast 14 and, in turn, the lamp 12.
As advantageous as the known dimming ballast systems have been in dimming and shutting down fluorescent lamps, there is a high required capital and labor cost associated with providing and installing the relay 22 and the power supply 26, together with their associated wiring. It would be desirable to eliminate such additional components and wiring, as well as to reduce the size of the overall circuit to save cost, weight and space and to provide greater efficiency.